1. Technical Field
The subject matter described herein relates to systems, apparatuses, and methods for ping-pong charge pumps.
2. Background Art
Internet technologies, such as cable modems and digital subscriber line (“DSL”) modems, often utilize charge pumps to drive signals on output lines. A charge pump is a voltage source that utilizes one or more capacitors to store charge. One type of charge pump used is a “ping-pong” charge pump. A conventional implementation of a ping-pong charge pump includes two flying capacitors that are coupled together by an equalization switch. The flying capacitors alternate (or “ping-pong”) between regulating an output voltage to drive output signals and charging, such that when one flying capacitor is regulating, the other flying capacitor is charging. In conventional implementations, the brief period of time when the flying capacitors alternate between regulating and charging is used for equalization of capacitor charges using an equalization switch.
A prior art representation of an equalization switch solution 100 is shown in FIG. 1. Equalization switch 102, typically a transistor such as the NMOS transistor shown in FIG. 1, is used to connect the top plates of the two flying capacitors during a brief equalization phase. A gate terminal 104 of equalization switch 102 is connected to an equalization clock signal φeq 122. A source terminal 106 of equalization switch 102 is connected to the top plate of a first capacitor having a voltage VT1 116, and a drain terminal 108 of equalization switch 102 is connected to the top plate of a second capacitor having a voltage VT2 118. A bulk terminal 110 of equalization switch 102 is biased to a voltage VBULK by a bias voltage source 114 (e.g., a unity gain buffer, as shown). Because a parasitic resistance 112 (RBULK) is present in routing metal wire, an extremely large bulk voltage VBULK may be induced by the bulk current (IBULK) during the operation of equalization switch 102 due to the voltage difference between VT1 116 and voltage VT2 118.
For instance, when equalization switch 102 is open (e.g., as shown when equalization clock signal φeq 122 remains low), the capacitors of the ping-pong charge pump are not connected, and each capacitor either charges (e.g., at level 124) or regulates (e.g., at level 126) the output voltage of the ping-pong charge pump. However, when equalization switch 102 is closed (e.g., as shown when equalization clock signal φq 122 transitions from low to high) the capacitors are directly connected through equalization switch 102, and their voltages VT1 116 and VT2 118 are equalized. When the difference in voltages VT1 116 and VT2 118 of the two flying capacitors is large at the time of closing equalization switch 102, snapback may occur. Snapback is a state in which high bulk current (parasitic current IBULK, as shown) flows through equalization switch 102 (e.g., due to parasitic resistance 112 (RBULK)) and can result in damage to and/or breakdown of the ping-pong charge pump. A snapback moment 120 is illustrated in FIG. 1 where the voltage difference between VT1 116 and VT2 118 is large. It should be noted, however, that even small voltage differences can cause a snapback state that may damage a ping-pong charge pump due to the bulk current IBULK and the parasitic resistance inherent in the routing metal wire (e.g., parasitic resistance 112 (RBULK) of routing metal wire).
Furthermore, conventional ping-pong charge pumps implement equalization switch solution 100, as shown, and require a large implementation area due to the equalization switch requirement.
Additionally, conventional ping-pong charge pumps are susceptible to voltage output ripple, a variance in the output voltage level of the ping-pong charge pump. High output ripple is often due to inconsistent voltages (i.e., voltage differences) between the flying capacitors caused by inadequate equalization times. For instance, as noted above, the brief equalization time (e.g., as shown when equalization clock signal φeq 122 transitions from low to high) available when the capacitors alternate between charging (e.g., at level 124) and regulating (e.g., at level 126) states is insufficient to prevent voltage output ripple.